1. Field of the Invention
This invention relates to a method for fabricating brazed aluminum fin heat exchangers with high tensile strength or bursting strength.
2. Description of the Prior Art
Recently, plants for extractive separation of useful components from natural gas and other hydrocarbon sources have been built in many parts of the world and a large number of pressure-resistant brazed aluminum heat exchangers have been employed in such plants. With the increased severity of operating conditions, such heat exchangers have had to be more pressure-resistant. Thus, the conventional brazed aluminum fin heat exchangers (for example, the equipment available under the trademark ALEX) have been fabricated by interposing a fin member between a pair of brazing sheets each made up of a core sheet and a cladding layer of brazing material on either side of said core sheet, immersing the assembly in a salt bath (about 600.degree. C.) to transform the brazing material into a semi-fused state and cooling it so that the fin and brazing sheets will be integrally joined. However, because AA 3003 (wherein AA signifies the standard of the Aluminum Association) brazing alloy is used in this prior art method, the maximum bursting strength has been only somewhere between 50 and 60 Kg/cm.sup.2, although the method is conducive to satisfactory fin formation. It has, therefore, been proposed to increase the thickness of the fin in an attempt to improve the bursting strength but such a procedure has proved to be detrimental to fin formation. Thus, it has been impossible to obtain small fins with a fin pitch of the order of several millimeters or what are known as serrated fins.
The aforementioned AA 3003 alloy may be replaced with AA 3004 in the manufacture of fins. Having a tensile strength beyond 17 Kg/mm.sup.2, AA 3004 provides for high bursting strength. However, this material has unsatisfactory fin formation characteristics. Moreover, AA 3004 contains nearly one percent of Mg and, in the course of the brazing operation, this element combines with Si in the brazing material (e.g., AA 4343). This interaction results in an increased diffusion of the brazing material into the fin material, thus interfering with the brazing. Thus, reduced strength and failures to establish brazed joints have been shortcomings of this process. Thus, by any of these prior art methods, it has been difficult to fabricate a brazed aluminum fin heat exchanger with bursting strength in excess of 80 Kg/cm.sup.2.
It has been proposed to prepare brazed aluminum fin heat exchangers using alloys of the Al-Mg-Si system wherein the brazed heat exchanger after brazing is subjected to a solution heat treatment followed by rapid quenching and subsequent age-hardening. Chartet, U.S. Pat. No. 3,852,873, discloses such a process in which the assembled heat exchanger is brazed and then immediately quenched at a rate of at least 1.0.degree. C./sec, that is at least 60.degree. C./min. and subsequently age-hardened. Moore, U.S. Pat. No. 2,837,450, discloses a similar process for making a heat exchanger, wherein the brazed exchanger is subjected to a solution heat treatment, quenched, and subsequently age-hardened. Burwen, U.S. Pat. No. 3,184,349, discloses a process for making a brazed aluminum enclosure for electronic devices which also involves subjecting the brazed article to a solution heat treatment, quenching rapidly with chilled gas, and subsequently age-hardening. All three of these patents disclose the use of alloys which contain little or no copper such as AA 6063. Burwen also discloses the use of AA 6061 which does contain copper, but his disclosure emphasizes the necessity of rapid quenching of the assembled brazed article after the solution heat treatment.
The rapid quenching of the assembled heat exchanger taught in these patents has a number of shortcomings in industrial practice. (1) Such rapid rates of quench are difficult to attain, especially for large heat exchangers which have an appreciable heat capacity. (2) The rapid contraction of the article during quenching can cause plastic deformation of the fin resulting in dimensional inaccuracies in the exchanger. The fin may even be ruptured by the stresses. (3) Residual stress due to the rapid quenching may remain in the heat exchanger which may give rise to stress corrosion cracking in use. (4) The strength of the fin may be nonuniform due to nonuniform quenching since the cooling rate may be different between the outside and inside surfaces of the fin.
The conflict between the requirement for rapid quenching to ensure proper hardening, and the problems introduced by the rapid quenching indicates that a need has continued to exist for a process capable of making a high-strength brazed aluminum fin heat exchanger using a slower rate of quenching in the heat treatment procedure.